The polymerisation of olefins in the gas phase in fluidised or similar reactors is well-known and commercially operated. In a typical fluidised bed process for the production of polyethylene, for example, a bed of growing polymer particles is maintained in a fluidised state in a vertically orientated cylindrical reactor by means of an ascending gas stream (fluidising gas). Gas exiting the top of the fluidised bed reactor is cooled and recycled to the base via a recycle loop. Fresh monomer(s) are added to replace those consumed in the reaction whilst produced polymer particles are continuously removed from the bed.
GB 1,413,613, for example, describes a process for the dry polymerisation of olefins in a fluidised bed reactor. Two possible configurations are described to address the potential problem of small solid particles, known as “fines”, being entrained from the reactor with the ascending gas stream. The first of these is the use of what is referred to in GB 1,413,613 as a “tranquilisation chamber” which is an area of larger cross-section compared to the reactor and provided above the reactor which acts to reduce the velocity of the fluidising gas, allowing solid particles which might otherwise exit the top of the fluidised bed to fall back to the bed. The second is the use of a cyclone to separate the entrained solids for subsequent return to the reactor.
Of the two options disclosed in GB 1,413,613, that referred to as the tranquilisation chamber, which may alternatively be, and, in fact, is more generally, referred to as a disengagement zone or velocity reduction zone, has become the commonly used means for reducing entrainment from fluidised bed reactors. However, despite the use of a disengagement zone, it is common for significant quantities of solids to remain entrained in the fluidising gas exiting the disengagement zone. Such solids may foul components of the line through which the fluidising gas is recycled, which can lead to a requirement for shut-down and cleaning. To reduce this problem it is therefore known to also use a cyclone in addition to the disengagement zone to separate entrained solids from the fluidising gas exiting the disengagement zone. Such processes are described, for example, in U.S. Pat. Nos. 4,882,400 and 5,382,638.
U.S. Pat. No. 4,882,400, for example, discloses an apparatus for the gas phase polymerisation of olefins in a fluidised bed reactor comprising a disengagement chamber and a cyclone which acts to separate entrained solids from the fluidising gas exiting the reactor, for recycle to the fluidised bed.
U.S. Pat. No. 5,382,638 also discloses an apparatus for the gas phase polymerisation of olefins in a fluidised bed reactor comprising a disengagement chamber and a cyclone which acts to separate entrained solids from the fluidising gas exiting the reactor, for recycle to the fluidised bed.
Cyclones are also known for essentially the same purpose for vapour phase polymerisation of propylene in horizontal gas phase reactors, for example as described in U.S. Pat. No. 4,640,963.
In general reaction (e.g. fluidising) gas passes through and exits the top of the reaction zone and is passed to a cyclone or other solids separation unit, wherein entrained solids are separated from the gas, usually for return to the reaction zone. In general the gas exits the reaction zone via a length of vertically orientated pipe at the top of the reaction zone and enters the cyclone, which is itself vertically orientated, at or near the top of the cyclone via a length of horizontally orientated pipe. Thus, the simplest connection is obtained by simply connecting the vertically orientated pipe to a length of horizontally orientated pipe which passes the gas directly to the cyclone. Further, a relatively high location for the cyclone has been considered advantageous in minimising back-pressure and providing a gravitational driving force for reinjection of the fines into the reactor. In particular, if it were just a case of “ease” of reinjection into the reaction zone, the solids would be reinjected above the fluidised bed. However, from a process point of view this can lead to direct re-entrainment of significant quantities of the reinjected solids. It is thus desirable to reinject the solids back into the fluidised bed itself, although this requires injection into a zone of higher pressure and hence requires a larger “driving force”.
Consistent with this, in the prior art documents mentioned above the inlet of the cyclone is shown located above the vertical height of the reactor outlet.